# Flat morphism

In mathematics, in particular in the theory of schemes in algebraic geometry, a flat morphism f from a scheme X to a scheme Y is a morphism such that the induced map on every stalk is a flat map of rings, i.e.,

fP: OY,f(P)OX,P

is a flat map for all P in X. A map of rings A → B is called flat, if it is a homomorphism that makes B a flat A-module.

A morphism of schemes f is a faithfully flat morphism if f is a surjective flat morphism.

Two of the basic intuitions are that flatness is a generic property, and that the failure of flatness occurs on the jumping set of the morphism.

The first of these comes from commutative algebra: subject to some finiteness conditions on f, it can be shown that there is a non-empty open subscheme Y′ of Y, such that f restricted to Y′ is a flat morphism (generic flatness). Here 'restriction' is interpreted by means of fiber product, applied to f and the inclusion map of Y′ into Y.

For the second, the idea is that morphisms in algebraic geometry can exhibit discontinuities of a kind that are detected by flatness. For instance, the operation of blowing down in the birational geometry of an algebraic surface, can give a single fiber that is of dimension 1 when all the others have dimension 0. It turns out (retrospectively) that flatness in morphisms is directly related to controlling this sort of semicontinuity, or one-sided jumping.

Flat morphisms are used to define (more than one version of) the flat topos, and flat cohomology of sheaves from it. This is a deep-lying theory, and has not been found easy to handle. The concept of étale morphism (and so étale cohomology) depends on the flat morphism concept: an étale morphism being flat, of finite type, and unramified.

## Properties of flat morphisms

Let f : XY be a morphism of schemes. For a morphism g : Y′ → Y, let X′ = X ×Y Y and f′ = f × g : X′ → Y. f is flat if and only if for every g, the pullback f* is an exact functor from the category of quasi-coherent ${\mathcal {O}}_{Y'}$ -modules to the category of quasi-coherent ${\mathcal {O}}_{X'}$ -modules.

Assume that f : XY and g : YZ are morphisms of schemes. Assume furthermore that f is flat at x in X. Then g is flat at f(x) if and only if gf is flat at x. In particular, if f is faithfully flat, then g is flat or faithfully flat if and only if gf is flat or faithfully flat, respectively.

### Fundamental properties

• The composite of two flat morphisms is flat.
• The fibered product of two flat or faithfully flat morphisms is a flat or faithfully flat morphism, respectively.
• Flatness and faithful flatness is preserved by base change: If f is flat or faithfully flat and g : Y′ → Y, then the fiber product f × g : X ×Y Y′ → Y' is flat or faithfully flat, respectively.
• The set of points where a morphism (locally of finite presentation) is flat is open.
• If f is faithfully flat and of finite presentation, and if gf is finite type or finite presentation, then g is of finite type or finite presentation, respectively.

Suppose that f: XY is a flat morphism of schemes.

Suppose now that h : S′ → S is flat. Let X and Y be S-schemes, and let X′ and Y′ be their base change by h.

• If f : XY is quasi-compact and dominant, then its base change f′ : X′ → Y is quasi-compact and dominant.
• If h is faithfully flat, then the pullback map HomS(X, Y) → HomS(X′, Y′) is injective.
• Assume that f : XY is quasi-compact and quasi-separated. Let Z be the closed image of X, and let j : ZY be the canonical injection. Then the closed subscheme determined by the base change j′ : Z′ → Y is the closed image of X′.

### Topological properties

If f : XY is flat, then it possesses all of the following properties:

If f is flat and locally of finite presentation, then f is universally open. However, if f is faithfully flat and quasi-compact, it is not in general true that f is open, even if X and Y are noetherian. Furthermore, no converse to this statement holds: If f is the canonical map from the reduced scheme Xred to X, then f is a universal homeomorphism, but for X noetherian, f is never flat.

If f : XY is faithfully flat, then:

• The topology on Y is the quotient topology relative to f.
• If f is also quasi-compact, and if Z is a subset of Y, then Z is a locally closed pro-constructible subset of Y if and only if f−1(Z) is a locally closed pro-constructible subset of X.

If f is flat and locally of finite presentation, then for each of the following properties P, the set of points where f has P is open:

• Serre's condition Sk (for any fixed k).
• Geometrically regular.
• Geometrically normal.

If in addition f is proper, then the same is true for each of the following properties:

• Geometrically reduced.
• Geometrically reduced and having k geometric connected components (for any fixed k).
• Geometrically integral.

### Flatness and dimension

Assume that X and Y are locally noetherian, and let f : XY.

• Let x be a point of X and y = f(x). If f is flat, then dimx X = dimy Y + dimx f−1(y). Conversely, if this equality holds for all x, X is Cohen–Macaulay, and Y is regular, then f is flat.
• If f is faithfully flat, then for each closed subset Z of Y, codimY(Z) = codimX(f−1(Z)).
• Suppose that f is flat and that F is a quasi-coherent module over Y. If F has projective dimension at most n, then f*F has projective dimension at most n.

### Descent properties

• Assume f is flat at x in X. If X is reduced or normal at x, then Y is reduced or normal, respectively, at f(x). Conversely, if f is also of finite presentation and f−1(y) is reduced or normal, respectively, at x, then X is reduced or normal, respectively, at x.
• In particular, if f is faithfully flat, then X reduced or normal implies that Y is reduced or normal, respectively. If f is faithfully flat and of finite presentation, then all the fibers of f reduced or normal implies that X is reduced or normal, respectively.
• If f is flat at x in X, and if X is integral or integrally closed at x, then Y is integral or integrally closed, respectively, at f(x).
• If f is faithfully flat, X is locally integral, and the topological space of Y is locally noetherian, then Y is locally integral.
• If f is faithfully flat and quasi-compact, and if X is locally noetherian, then Y is also locally noetherian.
• Assume that f is flat and X and Y are locally noetherian. If X is regular at x, then Y is regular at f(x). Conversely, if Y is regular at f(x) and f−1(f(x)) is regular at x, then X is regular at x.
• Assume again that f is flat and X and Y are locally noetherian. If X is normal at x, then Y is normal at f(x). Conversely, if Y is normal at f(x) and f−1(f(x)) is normal at x, then X is normal at x.

Let g : Y′ → Y be faithfully flat. Let F be a quasi-coherent sheaf on Y, and let F′ be the pullback of F to Y′. Then F is flat over Y if and only if F′ is flat over Y′.

Assume that f is faithfully flat and quasi-compact. Let G be a quasi-coherent sheaf on Y, and let F denote its pullback to X. Then F is finite type, finite presentation, or locally free of rank n if and only if G has the corresponding property.

Suppose that f : XY is an S-morphism of S-schemes. Let g : S′ → S be faithfully flat and quasi-compact, and let X′, Y′, and f′ denote the base changes by g. Then for each of the following properties P, f has P if and only if f′ has P.

• Open.
• Universally open.
• Closed.
• Universally closed.
• A homeomorphism.
• A universal homeomorphism.
• Quasi-compact.
• Quasi-compact and universally bicontinuous.
• Quasi-compact and a homeomorphism onto its image.
• Quasi-compact and dominant.
• Separated.
• Quasi-separated.
• Locally of finite type.
• Locally of finite presentation.
• Finite type.
• Finite presentation.
• Proper.
• An isomorphism.
• A monomorphism.
• An open immersion.
• A quasi-compact immersion.
• A closed immersion.
• Affine.
• Quasi-affine.
• Finite.
• Quasi-finite.
• Integral.

It is possible for f′ to be a local isomorphism without f being even a local immersion.

If f is quasi-compact and L is an invertible sheaf on X, then L is f-ample or f-very ample if and only if its pullback L′ is f′-ample or f′-very ample, respectively. However, it is not true that f is projective if and only if f′ is projective. It is not even true that if f is proper and f′ is projective, then f is quasi-projective, because it is possible to have an f′-ample sheaf on X′ which does not descend to X.